1. Field of the Invention
This invention relates to digital wireless communications and more specifically to methods and apparatus pertaining to an adaptive antennae weighting scheme.
2. Description of the Related Art
Digital wireless systems presently being deployed are commonly seen to be forward link (base station (“BS”) to mobile station (“MS”)) capacity limited. This is due mostly to asymmetric traffic: most data applications, such as web browsing, are envisioned to be sending data to the mobile user on the forward link, while the reverse link might contain only small control packets, i.e. a new IP address to download, so that the forward data rate is much greater than the reverse data rate.
Forward capacity can be increased with an array of antennae in a base station whose individual carrier amplitudes and phases can be adjusted based on feedback from a MS. In particular, a TxAA algorithm will typically transmit the same MS-specific waveform on multiple antennae, applying adaptive complex weights to the baseband signal applied to each antenna. To allow the MS to do coherent demodulation, a dedicated pilot channel will typically be transmitted in the same manner as the data.
While RxAA algorithms at the BS are fairly straight forward, TxAA algorithms are not. The mere definition of an “optimal” TxAA algorithm is not unambiguous as the optimization of one mobile's forward link can degrade another's, leading to complex tradeoffs which are not part of the RxAA problem.
Most TxAA algorithms will require some knowledge of the transmission channel from the BS to the MS. Given this knowledge and a defined “optimality”criterion, the BS can determine the TxAA weights. The difficulty here is that the MS can measure the channel, but it is the BS which needs this information to adjust its transmit weights. Also, there can be some additional complexity in that the mobile which must generally measure the channel of each transmit antenna separately, in addition to measuring the channel of the overall Tx weight adjusted signal, the latter being required for the demodulation of the signal.
Presently, there are a number of proposed methods for implementing TxAA. According to some of these proposals, a few bits are allocated to the MS to encode the channel gain and phase. See Thomas Derryberry, Balaji Raghothaman (Nokia) “Transmit Adaptive Arrays without User Specific Pilot”, document # C30-19990817-030, submitted to 3GPP2 August 1999; Mark Harrison (Motorola) “Tx AA Parameter Recommendations”, document #C30-19990914-010, submitted to 3GPP2, Tokyo Japan, September 1999; and Mark Harrison, Kiran Kuchi (Motorola) “Open and closed loop transmit diversity at high data rates on 2 and 4 elements”, document #C30-19990817-17, submitted to 3GPP2 August 1999. These methods do not allow the desired antenna weights to be precisely determined because the channel state must be distorted in order to be fed back to the BS with a low bit rate.
The academic literature has typically assumed that the full channel information is available at the transmitter, which is not a practical assumption. See, e.g. Jen-Wei Liang, Arogyaswami Paulraj “Forward link antenna diversity using feedback for indoor communication systems” Proceedings, 1995 International Conference on Acoustics, Speech, and Signal Processing, May 1995; Farrokh Rashid-Farrokhi, K. J. Ray Liu, Leandros Tassiulas “Transmit beamforming and power control for cellular wireless systems” IEEE Journal on Selected Areas in Communications, Vol. 16, No. 8 Oct. 1998. There have been several submissions to the TIA standardization body for TxAA algorithms with channel feedback from MS to BS, some of which are referenced above. All of these submissions require the mobile to measure a primary and secondary pilot transmitted from the primary and secondary antennae. Some submissions have shown that 4 antenna transmission can give better performance, with no recognition of the increased complexity at the MS.
The use of a few bits of channel feedback leads to substantial degradation relative to the possible performance, since the feedback requires low bit rate quantization. Also, the mobile must individually measure and transmit information for each antenna; the MS requires extra hardware to perform these functions. Furthermore, the above described methods do not gracefully grow to more antennae.
The above mentioned systems are illustrated in FIG. 1, FIG. 2 and FIG. 3, which illustrate a CDMA system. FIG. 1 shows a transmitter 10 with two antennae, first antenna 12 and second antenna 14. As is shown, different common pilot signals are associated with different antennae. For each user, such as user 0 and user 1, the transmitter 10 includes an adder, 16 and 18 respectively, that adds together a dedicated pilot signal for the user and the forward traffic for that user. Multipliers 20 and 22 multiply the summed signal with complex weights for the first and second antennae 12 and 14, respectively. For each antenna, the weighted user signals are summed by adders 24 and 26 and the result is added by adders 28 and 30 to the pilot signal for that antenna. (It should be noted that FIG. 1 represents a complex baseband equivalent, as no RF modulation stage is shown.)
FIG. 2 and FIG. 3 show possible embodiments for receivers that may be used to receive signals transmitted by the transmitter 10. As shown in FIG. 2, a received signal is divided into three components: one signal corresponding to the dedicated channel, one signal corresponding to the common pilot for the first antenna and one signal corresponding to the common pilot for the second antenna. This division is accomplished by multipliers 32, 34 and 36 and accumulators 38, 40 and 42. A more hardware efficient way is shown in FIG. 3, which employs multiplexer 44 and demultiplexer 46 to alternately select between the signals for the different transmit antennae. This time multiplexed processing saves hardware at the expense of 3dB loss of precision for each channel, which is acceptable given the low bit rate, low precision channel reporting used by the mobile to report these channel estimates to the BS.
The channel estimate the mobile attains is coded into some low bit representation. The bit rates mentioned in the above cited references are 1,2 or 4 bits (1b phase, 2b phase, or 3b phase+1b amplitude). So, for a forward channel vector c, the mobile generates the estimate ĉ, which is then quantized to give the feedback estimate .
Note that the MS channel estimation hardware of FIG. 2 or FIG. 3 can be used for an arbitrarily large number of Tx antennae, as long as there are unique pilot codes for each antenna and MS knows all of these codes.
Finally, the proposed systems have the MS report the channel estimate based on 1 “path”. In the presence of resolvable multi-path due to delayed reflections of the transmitted waveform, particularly for CDMA, there may be more than one path usable to the MS. In order to report the channel for N such paths, the MS to BS feedback rate would have to increase N-fold, and the number of such paths would somehow have to be communicated to the BS. This is not practical, and instead the MS reports the channel estimate for only the strongest path. This discards some useful channel characteristics which could further increase performance under these circumstances.
The algorithm employed by the BS to utilize the received channel information would most likely be a simple matched transmission weighting. That is, the forward weights chosen would be the conjugate of the forward vector channel, so that the weights arew=*
This formulation maximizes the signal power to the mobile without regard to the locations of the other mobiles. It does not steer nulls to the other mobiles. The channel estimate from a given mobile could be used to determine transmission nulls of the other mobiles, but the coarseness of the channel estimate (no more than 4 bits) makes this ineffective.
To summarize, schemes such as that shown in FIG. 1, FIG. 2 and FIG. 3 have significant drawbacks. In particular, as previously mentioned, the use of a few bits of channel feedback requires quantization which leads to substantial degradation relative to the possible performance. Also, since an MS unit must individually measure and transmit information for each antenna; the MS requires extra hardware to perform these functions. Furthermore, the above described methods do not gracefully grow to more antennae.